Marine pilings and other equipment are subject to constant attack by salt water, erosive forces by waves, and the constant threat of biofouling and the damaging effects of marine growth. All of these severe forces limit the lifespan of marine equipment of all kinds.
According to some estimates, over 1700 species comprising over 4000 organisms are responsible for biofouling. Biofouling is divided into two types. Microfouling—biofilm formation and bacterial adhesion—and macrofouling—attachment of larger organisms (see e.g., FIG. 1 showing the various layers of biofouling as well as damage to the support structure). Due to the distinct chemistry and biology that determines what prevents them from settling, organisms are also classified as hard or soft fouling types. Calcareous (hard) fouling organisms include barnacles, encrusting bryozoans, mollusks, polychaete and other tube worms, and zebra mussels. Examples of non-calcareous (soft) fouling organisms are seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a fouling community and cause serious damage, including infiltration of the equipment with roots and other attachment systems, contribute to excess weight and frictional drag, and can lead to catastrophic equipment failure. Indeed, failure of the Alexander Kielland and Ocean Ranger oil production platforms has been attributed in part due to biofouling, both from increased load and windage, as well as from damage that was hidden under the thick fouling layer.
For example, the degradation of wooden pilings and docks by wood-boring organisms in marine environments has been a well-known problem for centuries. The Limnoria Tripunctata (crustacean borer) and the Teredinidae (borer shipworm, a bivalve borer) are two of the most common destructive organisms found in US regional waterways, as both types of borers attack wood for shelter and food. These organisms are responsible for hundreds of millions of dollars in damage to wooden marine structures. In fact, the life expectancy of an unwrapped chromated-copper-arsenate and/or ammoniacal-copper-zinc-arsenate “pressure-treated” piling that is fully exposed to the elements is only seven to ten years.
Even steel and concrete pilings are subject to considerable wear-and-tear in a marine environment. The spray and splash zone above the mean high tide level is the most severely attacked region due to continuous contact with highly aerated sea water and the erosive effects of spray, waves and tidal action. Steel corrosion rates as high as 0.9 mm/year at Cook Inlet, Alaska, and 1.4 mm/year in the Gulf of Mexico have been reported. Cathodic protection in this area is ineffective because of lack of continuous contact with the seawater, the electrolyte, and thus no current flows for much of the time. Corrosion rates of bare steel pilings are often also very high at a position just below mean low tide in a region that is very anodic relative to the tidal zone, due to powerful differential aeration cells which form in the well aerated tidal region.
Barnacle accretion is reduced on concrete pillars, as compared with steel pillars (see FIG. 2). However, concrete has other difficulties. Concrete exposed to marine environment may deteriorate as a result of combined effects of chemical action of seawater constituents on cement hydration products, alkali-aggregate expansion (when reactive aggregates are present), crystallization pressure of salts within concrete if one face of the structure is subject to wetting and others to drying conditions, frost action in cold climates, corrosion of embedded steel in reinforced or prestressed members, and physical erosion due to wave action and floating objects.
Thus, fouling, erosion and corrosion causes significant structural damage to marine equipment, such as pilings. Furthermore, the weight and loadings that result from fouling can be so significant as to necessitate considerable ‘over-design’ of such structures compared to what would otherwise be required.
Thus, marine equipment must be periodically inspected and cleaned to keep biofouling to a minimum. High pressure water blasters and handheld scrapers have proven to be effective in-water tools for the removal of fouling from a range of structures (including oil rigs). However, such methods are tedious and require considerable manpower and/or submersible equipment, both of which are expensive. Dry-docking and land-based cleaning methods such as sand blasting are the other option, but obviously this requires considerable downtime, and can be impractical for many nearshore and offshore oil exploration and production equipment.
Another option is to provide protective coatings for marine pilings. Cuprotect®, for example, is a polyurethane coating that can be applied to a piling like paint. However, coatings have a limited lifespan, and once the coating has lost efficacy, the problem reemerges.
Another potentially effective method is plastic wrapping. Vessels in many size categories, as well as artificial structures (e.g. wharf piles, moorings, fish farming cages) have been treated in situ by encapsulating them in plastic wrapping. The method relies on the development of anoxic conditions in the encapsulated water and, if necessary, mortality can be accelerated through the addition of non-persistent chemical agents (e.g. acetic acid and bleach). However, the practicality and efficacy of this concept being applied to a marine structure of the dimensions of a semi-submersible drilling rig is yet to be established by any detailed research or experimentation. Further work is also required to clarify the factors that influence mortality rates (e.g. temperature, fouling biomass) so that informed treatment guidelines can be developed.
Other plastic wrap methods have been proposed. U.S. Pat. No. 2,724,156, for example describes a single layer tough flexible waterproof pole boot. U.S. Pat. No. 5,180,531 describes extruding a continuous, substantially homogeneous plastic layer at least two inches thick on substantially the entire length of a steel core of a piling.
U.S. Pat. No. 7,300,229 describes a repair jacket for spot repair of a piling which includes a cylindrical body of fiber-reinforced plastic (FRP) material that is about 0.5 to 3.5 inches greater in diameter than the piling. The body is wrapped around the piling, sealed and filled with expanding grout to create a rigid seal at the bottom of the gap. Similarly, U.S. Pat. No. 7,871,483 describes using a plastic shrink wrap for spot repair.
A high density polyethylene material that is 0.030″ thick and is available in both 36″ and 60″ widths is available to wrap wooden pilings. However, the pile is wrapped using roofing nails, and for best results, the manufacturer instructs that nails should be installed every 2″ along the seams. Thus, this piling is not easily removed, and is usable for only a single protective session.
Similarly, U.S. Pat. No. 6,872,030 describes a composite wrapping, formed on the piling by a filament winding process. Filament strands are impregnated with resin and wrapped around the wood piling under tension. The resin is allowed to cure to form a seamless layer which is uniform in thickness and materials. This method is fairly complex, and thus impractical for larger marine structures more complicated than a simple pole. Further, it cannot be easily applied to existing structures.
Another solution is to replace wooden pilings with other materials, less susceptible to biofouling. For example, fiberglass pilings are available and are impervious to any borers and worms. However, barnacles and other marine organisms are still a problem, and are still difficult to remove if left for any length of time. Further, as noted above, even steel and concrete pilings are subject to extreme wear in a marine environment.
None of the above solutions address the need for a simple easy way of cleaning pilings, without having to pull the pilings to dry dock for maintenance or spend significant amount of dive time cleaning the pilings underwater. Any method can reduce the frequency at which pilings are dry docked or cleaned in situ would be of tremendous cost and time savings.